1. Field of the Invention
The present invention relates to an optical wave guide and an optical input device, and fabrication methods thereof. The present invention also relates to a liquid crystal display apparatus which uses the optical wave guide and the optical input device fabricated by the methods.
2. Description of the Related Art
As planer-type display apparatus, an electroluminescence panel (ELP), a plasma display panel (PDP), a liquid crystal display (LCD) and the like are developed. Among them, the LCD can readily attain a full-color display, and it can readily be connected to a large scale integrated circuit (LSI) without using a special interface. Accordingly, the LCD is the most promising as the planer-type display apparatus, and the technology related to the LCD is remarkably advanced. In recent years, methods in which optical signals are used for signal transmission has been investigated for the LCD, in order to eliminate problems such as signal delay caused by an increase in size of the display apparatus, an increase in the number of pixels to be driven, or the like.
If such optical signals are used for signal transmission, an optical input device is required. The optical input device is used for transmitting light as a signal from a light source to a predetermined portion and for inputting the light. In a conventional optical input device, an optical fiber or an optical wave guide formed on a substrate is utilized.
An optical wave guide is formed on a quartz substrate, on a multicomponent glass substrate or on a plastic substrate. In the case where the optical wave guide is formed on the quartz substrate, a flame deposition method is generally used. In the case where the optical wave guide is formed on the multicomponent glass substrate, an ion exchanging method is generally used. According to both the above methods, the optical wave guide can be formed for either a single mode or a multi mode. In the flame deposition method, a silane gas as a material gas is subjected to a combustion reaction so as to deposit it on the substrate. In the ion exchanging method, a molten salt including ions of Ag, Tl, K or the like as a ion source are diffused in soda lime or borosilicate glass by a heating treatment. Alternatively, in the case where the optical wave guide is formed on the multicomponent glass substrate, a sputtering method can be used.
On the other hand, methods for forming an optical wave guide on a substrate of plastic which is an organic material include a selective photopolymerization method, a photo-locking method or the like. In the selective photopolymerization method, polycarbonate as a base material and methyl acrylate as a monomer are selectively exposed to ultraviolet light, so as to perform polymerization. The refractive index of the monomer is lowered by the polymerization, so that the unexposed portion becomes an optical wave guide with a high refractive index.
In addition, an LCD in which optical signals are used for signal transmission necessitates a transistor which performs switching by detecting an input light beam from the optical input device. Conventionally, a pn phototransistor is employed. Regarding a material for photoelectric transduction in the pn phototransistor, single-crystal Si or amorphous Si (hereinafter, referred to as "a-Si") is used for detecting light in a visible light range, and Ge, lead calcogenide, or the like is used for detecting light in an infrared range.
Moreover, when optical signals are used in the LCD for signal transmission, it is necessary to determine the locations of the light source and the optical input device. In a conventional LCD, one light source is provided for each of a plurality of optical input devices which are disposed in parallel to each other in a row or column direction.
In the case where an optical fiber is utilized as an optical input device, there arises a problem in that the connections to other optical components are complicated. For simplifying the connections, there is an attempt to utilize an optical wave guide formed on a substrate as the optical input device. However, this case still has a problem in that an input/output scheme suitable for the optical wave guide has not yet been found.
As described above, the optical wave guide is formed on the quartz substrate by the flame deposition method or formed on the multicomponent glass substrate by the ion exchanging method. However, conventionally, even in a linear optical wave guide, the optical loss has a large value, i.e., 0.1 dB/cm. In a curved optical wave guide, the optical loss has a further increased value. Therefore, the conventional optical wave guide having a length of 10 cm or more cannot be practically used. The reason why the optical wave guide has an optical loss larger than in the case of the optical fiber is that the interface between a side face of the optical wave guide and the substrate may be roughed. For example, if there exists a rough portion having a difference in level of about 100 .ANG. on the side face of the optical wave guide having a diameter of 1 .mu.m, about 5% of light intensity is scattered and lost from this portion. Such a rough portion on the side face inevitably occurs by the conventional fabrication method of the optical wave guide. Therefore, in cases where the optical wave guide is used, it is necessary to reduce the optical loss to a minimum by concentrating the light on a center of wave guide so that the light intensity on the side face is decreased.
Optical loss in the optical wave guide is also caused by nonuniformity in refractive index along the propagation direction. For example, it is assumed that a fiber having a circular section with a diameter of 100 .mu.m has a step portion as a nonuniform portion having a diameter of 110 .mu.m and a length of 100 .mu.m. Even when the variation refractive index is 1% at most, 5% of total light amount is scattered from the step portion, which proves that the nonuniformity severely affects the optical loss. Therefore, in order to equally distribute light to 3000 or more pixel electrodes in the LCD by means of the optical wave guide, a working accuracy in the order of 1 .mu.m is required.
In the LCD, in order to prevent liquid crystal from being deteriorated, it is necessary to periodically reverse the polarity of voltage applied to the liquid crystal during the drive operation. For this purpose, the current direction is periodically reversed when electric charges are to be stored in a capacitor constituted by a pixel. A conventional pn photodiode for detecting light has a rectifying function, so that current cannot flow in the reverse direction to the pn junction. Accordingly, when a conventional pn photodiode is used for driving pixels, the amount of current largely varies depending on the current direction. This results in that a time period required for storing electric charges to a capacitor constituted by a pixel varies depending on the current direction, whereby there arises a problem in that the control is complicated.
With a construction in which a plurality of optical wave guides are connected to a single light source, the number of light sources can be reduced and the LCD construction can be simplified. However, such construction necessitates means for distributing light from the single light source to the plurality of optical wave guides. Unless the distributing means can be formed in a simple structure, the LCD still has a complicated construction.